Many complex metal structures are assembled by brazing, a process of joining two metal surfaces by introducing a third metal, the brazing alloy or filler metal, between them at an elevated temperature. When the elevated temperature is relatively modest, e.g. below about 500.degree. C., the process may be called soldering. Upon cooling, the two original surfaces, along with the filler alloy, are metallurgically bonded into a unitary structure. Often it would be very desirable to be able to reverse this process, i.e. remove the filler metal and allow the two component surfaces to be separated for repair or salvage. However, merely heating the structure again to the brazing temperature will not cause the filler alloy to simply flow out of the joint but instead may cause the bond to be strengthened by diffusion of the braze alloy into the components.
While the components may be made of almost any material, including ceramics, the components of most interest in this invention are made from nickel, cobalt, and iron based stainless steels and superalloys commonly used, for example, in the aerospace industry for gas turbine engine subassemblies. Such components are very expensive and worth salvaging for repair and reuse. Common brazing alloys used with these base metals include silver, copper, gold, and nickel based alloys.
One method currently used to separate brazed components is to soak them in a warm chemical bath until the braze alloy is chemically leached from the joint. However, such a process is often complex, very time consuming and frequently requires ultrasonic agitation, and the chemical media, which is often a strong acid, can cause damage to the components and/or the environment. See, for example U.S. Pat. No. 4,274,908 to Fishter at al. which discloses a complex nitric acid solution for removing gold--nickel braze alloy from superalloy parts; U.S. Pat. No. 4,302,246 to Brindisi et al. which discloses improved acid solutions; and U.S. Pat. No. 4,324,626 to McGivern which disclosed an electrolytically assisted acid leaching process.
It should be apparent from the foregoing that there has been a long felt need in this art for a more efficient process for disassembling a brazed structure without damage to its components.